H2O flow

Rainwater Site Flow Map & Volume Calculations for design project site: 

• Water running off from higher elevations to the south by southwest of the property enters primarily from the southwest corner of the site and leaves the site largely to the northeast, running in a diagonal direction from one corner of the property, flowing around the house, and flowing out the opposite corner of the property.
• The major impermeable surface of the site is the main structure (residential house).  This square footage is approximately 650 sq ft (tiny!), and this drains off of asphalt/ shingled roof material (runoff coefficient is 90%).  The water flows off of impermeable surface through gutters to the four corners of the house, with the largest volume of water flowing out the gutter on the northwest side of the house. 
• Volume of water is draining off of impermeable surfaces at the discharge locations:  Average annual rainfall: 54.52 in/ yr.  For my calculations I am using 55 in/ yr.
  1 cubic ft of water = 7.48 gallons
  55in/12 in = 4.583 ft of rain per yr
  --->  4.583 ft/yr * 650 sq ft = 2979.17 cubic ft of water that runs off the roof per yr
  --->  2979.17 cubic ft/ water * 7.48 gallons/ cubic ft = 22282.55 gallons
  --->  22282.55 gallons * 90% (runoff coefficient of asphalt/shingle roof) =  20054.29 gallons of runoff water that could potentially be harvested off the roof per year